My guess? Malthael brings to the council and you information that his job is going in to overtime due to the folks dieing. Everyone agrees to send you down to retrieve the crystal and to work together with Tyreal and the new Horadrim to seal it permanently. Now, its obvious we do retrieve the crystal and its obvious Tyreal attempts to seal it. And from the video it looks like for some reason or another Malthael decides to take it for himself.
My theory? Given this is from Blizzard and Blizzard has shown a lean towards WoW's expansion system for Diablo 3... what if the launch title does not have us fight Malthael? What if he dosent show up until mid way through the expansion? What if Act 5 leads up to Malthael's betrayal and... udunno maybe an Act 6 a year down the road as a big patch leads to him? I'd like this alot to be honest. Lets compare how a WoW expansion and how up until now a Diablo expansion worked.
WoW. Expansion hits, core mechanics in place, new places to run around, entry content for new expansion available. Every 3 months, new content added on to expansion. 1 year in to expansion final boss shows up via patch with related content and shortly there after next expansion announced and players only wait 1 year for new content.
Diablo series. Expansion hits, all content present including final boss. Players wait 2 years for new expansion.
I'd very much like to see WoW's expansion system applied to the Diablo franchise. Content at launch and periodic content added over course of two years until next expansion. Sounds tastier that way heh. Anywho, enjoy the trailer below, its the cinematic one and it is awesome.